1. Field of the Invention
The present invention relates to an arrangement and a method for detecting so-called double talk in a long distance telephone communications system, and more specifically to such an arrangement and a method which effectively prevent degradation of echo cancellation by rapidly detecting double talk.
2. Description of the Prior Art
A long distance telephone communications system has been constantly plagued by "echo". A known approach to solving this problem is the use of an echo canceller or echo suppressor using an echo estimator which takes the form of a digital transversal filter.
However, when both parties talk simultaneously, i.e., during double talk, the echo canceller is no longer able to effectively block echo signals. This is because the echo signals are included in near-end party's signals to be transmitted. Accordingly, it is a current practice to provide a double talk detector. More specifically, when the double talk is detected, updating of tap-coefficients of the transversal filter is terminated in order to prevent the echo cancellation from being undesirably lessened.
Before turning to the instant invention, it is deemed advantageous to briefly discuss a known echo canceller, which includes a double talk detector, with reference to FIG. 1. An arrangement similar to the echo canceller shown in FIG. 1, has been disclosed in Japanese Patent Application published under publication No. 64-27325 on Jan. 30, 1989.
As shown in FIG. 1, the known echo canceller denoted by reference numeral 10, includes an incoming signal memory 12 which is arranged to memorize an incoming signal IN applied via an input terminal 14. The incoming signal IN is also applied to a 4-wire to 2-wire hybrid circuit 16. An estimated echo signal generator 18, taking the form of digital filter (viz., convolution arithmetic circuit), is supplied with the incoming signal IN stored in the memory 12 and also supplied with tap coefficients from a tap coefficient memory 20. A subtracter 22 receives an echo signal EC routed towards a far end and further receives an estimated echo signal ES from the estimated echo signal generator 18. The subtracter 22 subtracts out the echo signal. A tap coefficient updating circuit 24 receives an error signal (viz., residual echo signal) ER outputted from the subtracter 22, the current tap coefficients from the memory 20 and the incoming signal IN stored in the memory 12, and then updates the tap coefficients stored in the memory 20 in order to render the error signal ER zero. During the double talk, a near end party's signal is added, by way of a terminal 15 and the hybrid circuit 16, to the echo signal EC. An echo signal and/or a near end party's signal are applied to a far end equipment through an output terminal 23.
During the double talk, since the near-end party's signal is added to the echo signal EC, the error signal ER undesirably increases and accordingly disturbs the tap coefficients. This means that an effective echo cancellation is no longer expected. In order to overcome such a problem, a double talk detector 30 is provided to ascertain or detect the double talk and terminates the updating of tap coefficients at the circuit 24.
The double talk detecting unit 30 includes an echo signal power detector 32, an error signal power detector 34, two comparators 36, 38, and an incoming signal power detector 40, all of which are coupled as shown. The power detector 32 is coupled to detect signal power of the echo signal EC, which is denoted by EC'. On the other hand, the power detectors 34, 40 are coupled to detect each power energy of the error and incoming signals ER and IN respectively, and produce the outputs ER' and IN' which respectively indicate the power of ER and the power of IN. In the event that the signal power EC' is larger than the signal power IN', the comparator 36 outputs a signal DT1 indicative of the double talk. The tap coefficient updating circuit 24 terminates the operation thereof in response to the signal DT1. Similarly, if the signal power ER' is larger than the signal power IN', then the comparator 38 outputs a signal DT2 indicative of the double talk. The tap coefficient updating circuit 24 also terminates the operation thereof in response to the signal DT2. The double talk detector 30 can be modified in a manner which responds to one of the two signals DT1, DT2.
However, the aforesaid known technique has encountered the problem in that the signal power detection requires a somewhat long time duration and hence it is practically impossible to detect immediately after the double talk occurs. Accordingly, the tap coefficients are disturbed and hence the echo cancellation is lowered with degrading speech quality. Further, the above-mentioned prior art is also unable to detect an instantaneous noise involved in the echo signal, and hence speech quality is also undesirably lowered.